1. Field of Invention
The present invention relates to a demodulator circuit in a communication system using a multi-carrier modulation scheme. The invention relates in particular to the effective demodulation of a received signal in a COFDM demodulator for a digital television receiver.
2. Related Art
Recently there has been a rapid development in the field of digital television broadcasting following the establishment of the European Digital Video Broadcast standard for digital terrestrial television (DVB-T) developed by the Digital Video Broadcasting Group.
In accordance with the DVB-T standard, a number of carrier frequencies are provided and data to be transmitted is spread over a large number of orthogonal data carriers using Coded Orthogonal Frequency Division Multiplexing (COFDM).
Each carrier can be encoded to carry a symbol containing more than one bit, for example by using a rectangular constellation modulation system such as 16-QAM, as is known to a skilled person.
An exemplary 16-QAM constellation diagram is shown in FIG. 1. As is known to a skilled person, each point on the 16-QAM constellation diagram corresponds to a 4-bit symbol. The symbols are normally assigned to the constellation points using Gray coding, in which symbols with similar most significant bits are grouped together.
At the transmitter the carriers are modulated in accordance with successive symbols to be transmitted and each received signal is demodulated in the receiver to the corresponding symbol using the constellation diagram. In most cases the signal received will not correspond exactly with a constellation point because of interference or noise in the channel between the transmitter and the receiver. In this situation the receiver must demodulate the received signal to the symbol corresponding to the constellation point which is most likely to have been transmitted.
It is known to de-map signals using soft decision decoding in which, instead of a “hard” decision as to whether a bit should be decoded as a “1” or as a “0”, a soft decision, comprising the “hard” decision and an indication of the level of confidence to be placed in the decision is output.
In simple systems the level of confidence which can be placed on the demodulated or de-mapped information is proportional to the distance or separation of the received signal from the expected constellation point. Clearly, the closer the received signal is to a constellation point, the more confidence can be placed in the de-mapped symbol.
When Gray coding is used, the level of confidence that can be placed in a particular de-mapped bit varies from bit to bit within the symbol.
The information from the de-mapper is passed to a viterbi decoder which decodes the bits.
This soft decision information can be input into a soft decision Viterbi decoder. A soft decision Viterbi decoder maintains a history of many possible transmitted sequences and builds up a view of their relative likelihoods. The Viterbi decoder selects a ‘0’ or a ‘1’ as the decoded bit based on the maximum likelihood. In this way the Viterbi decoder can exploit information relating to the expected reliability of each bit based on the proximity of each bit to the expected constellation point.
One problem with television broadcasting is the existence of multi-paths arising either as a result of the reception at the receiver of multiple copies of the signal emitted from a single transmitter, or as a result of the reception of signals from a number of transmitters all broadcasting the same signal. In the frequency domain, the existence of multi-paths is equivalent to a frequency selective channel response.
Furthermore, in situations where conventional analog television signals are transmitted within or overlapping the frequency range used by the digital television signal, the conventional analog television signals act as narrow interfering signals within the signal bandwidth of the digital television signal.
This frequency selective channel response characteristic results in the large number of different carriers used in COFDM modulation having different signal to noise ratios (SNR). Clearly, data conveyed by carriers having a high SNR is likely to be more reliable than data conveyed by carriers having a low SNR.
An estimate of the SNR of each carrier made by the receiver is called the channel state information (CSI) for the channel represented by that carrier. FIG. 2 illustrates a typical variation in carrier CSI for a COFDM signal with co-channel analogue television interference.
One known method of establishing channel state information for a COFDM signal is disclosed in the article “A demapping method using the pilots in COFDM system” IEEE Transactions on Consumer Electronics, Vol 44 No. 3 August '98 pp 1150-1153. This method utilizes the fact that pilot carriers with known magnitudes are transmitted with the COFDM signal, for equalization purposes. An estimate is made of the mean square error in the magnitude of the received pilot carriers and channel state information in the pilot carrier positions can be obtained from this estimate. The channel state information in useful data positions can be obtained by subsequent interpolation between the values calculated at the pilot carrier frequencies.
In order to provide robust performance of the system in an environment having a frequency selective channel response, it is known to use the channel state information in the Viterbi decoder when decoding the bits in order to provide extra information regarding the reliability of the bits based on the signal to noise ratio of the carrier.
The article “Performance analysis of Viterbi Decoder using channel state information in COFDM System” IEEE Transactions on Broadcasting, Vol 44 No. 4 15 December 1998 pp 488-496, describes a Viterbi decoder which uses Channel State Information calculated from a mean square estimation of the received pilot carrier signals in a COFDM system, to affect the Viterbi decoder branch metric values used to decode 3 or 4 bit soft decision data.
Previously it has also been suggested that if the channel state information of a particular channel is sufficiently bad, it can be concluded that no reliance can be placed on the data received on that channel. As a result, the Viterbi decoder may effectively record that no information is available regarding that bit by disregarding, or “puncturing” the corresponding bit or bits.
The transmitted data is coded using a convolutional code, which introduces redundancy in the signal in order to allow error correction of the signal to be achieved. The effect of the puncturing of data bits in the Viterbi decoder as indicated above, is merely to reduce the effective code rate of the signal. If a sufficiently robust code is used, the effective reduction in code rate resulting from the puncturing of bits can be tolerated, thus avoiding an impact on the decoded signal quality.